1. Field of the Invention
This invention relates to an image forming apparatus using an electrophotographic method such as a photocopier and a laser beam printer.
2. Description of Related Art
Various methods have been proposed and produced these days as multicolor image forming apparatuses using the electrophotographic image forming process. For example, exemplified are a multiple transfer method in which images in respective colors are sequentially formed on a transfer material carried on a transfer material carrying member and in which those images are fixed after completion of formation of images in all colors, and a one-time transfer method in which images in respective colors are transferred to an intermediate transfer member once and in which the images in all colors are transferred at a time and fixed to a transfer material after Completion of image formation of all colors done to the intermediate transfer member. Furthermore, there is a method for doing a transfer process to a transfer material or an intermediate transfer material as a one process where image forming units for all the colors are arranged in parallel with respect to the transfer material carrying member or the intermediate transfer member. Those selections are properly chosen with respect to the targeted specifications such as the size, costs, and printing speed of the image forming apparatus.
FIG. 5 is a schematic cross section showing an image forming apparatus most adapted to personal users among the image forming apparatuses described above realizing a compact size. It is structured such that image forming units are surrounding a photosensitive unit as a first image bearing member as a center. Particularly, an intermediate transfer member serving as a second image bearing member realizes a compact apparatus by integrating with the photosensitive unit and accomplishes improvements in usability by reducing the replacement unit number.
As a method for transmitting drive, the drive for units 130 (photosensitive unit, intermediate transfer unit) detachably attached to an image forming apparatus is performed by jointing a coupling member arranged at the respective units with respective coupling members held at the image forming apparatus.
FIG. 6 is an illustration showing a drive shaft of a photosensitive drum and a coupling member C1 formed at a shaft end of a belt-driven roller; (a) is a cross section showing a fitting state of the coupling member C1 and the coupling member C2 of the image forming apparatus when viewed from a lateral side; (b) is a a cross section showing an isolated state of the coupling member C1 and the coupling member C2 of the image forming apparatus when viewed from the lateral side; (c) is a perspective view showing a coupling member C1.
In FIG. 6, the coupling protrusion C1 of the drive shaft (the shaft of a photosensitive drum and a belt drive roller) and the coupling recess C2 of the image forming apparatus are in the fitting state during attachment of the coupling members, and the photosensitive drum 101 and the intermediate transfer belt 105 are driven by transmitting thereto drive applied by a driving means, not shown, of the image forming apparatus.
Where the unit is detached, the coupling member of the image forming apparatus is moved in arrow C direction, and where the coupling members are isolated, the unit is detached. This detaching operation can be done with incorporating, e.g., a mechanism associated with open and closed operation of a door of the image forming apparatus. When the image forming unit is attached, the coupling member of the image forming apparatus is moved in a direction opposite to arrow C in utilizing substantially the same jointly operating mechanism, and the drive force is transmitted to each unit upon fitting of the coupling protrusion and recess according to rotation of the coupling members in association with the beginning of the image forming operation.
Where the photosensitive body unit and the intermediate transfer unit are isolated, the photosensitive drum 101 and the intermediate transfer belt 105 can be kept easily in an isolated state, by providing a mechanism releasing an urging means, not shown, for urging the transfer material 150 toward the intermediate transfer belt 105.
However, where the photosensitive body unit and the intermediate transfer unit are integrated, it is difficult to provide a mechanism keeping the photosensitive drum 101 and the intermediate transfer belt 105 in an isolated state. Even where such a mechanism is provided, the mechanism likely has a complicated structure.
To realize image forming apparatuses for personal use aiming at being made further compact even a little, it is disadvantageous to make compact where the structure is complicated, and such a complicated structure brings increase of the part number, which is very unfavorable in terms of reduction of costs.
Accordingly, the structure, which is simple and can be handled easily by the users, is an image forming apparatus having no isolation mechanism for the photosensitive unit and the intermediate transfer member unit and being in a state that the photosensitive unit is always in contact with the intermediate transfer member unit.
However, with such a structure, the following may occur. (a) Where a photosensitive body unit detachably attached to an image forming apparatus and an intermediate transfer member unit are attached to an image forming apparatus body, differences among the period that the coupling is completed at respective units may occur. This is due to shapes of the coupling members or coupling phase differences of the coupling members when the coupling members are fitted. Those time differences appear as differences in timings for beginning driving at the respective units. The timing difference in beginning driving caused by the phase difference is of a phase difference of 120 degrees as the maximum value in a case of the triangle coupling members as shown in FIG. 6, and the time difference to nullify the difference becomes the differences in timings to begin driving.
Where the photosensitive body unit first drives and then the intermediate transfer belt begins driving according to timing differences of driving beginning, the drive torque of the photosensitive body unit first beginning the rotation is varied according to the drive of the intermediate transfer belt, thereby generating irregular rotation in the photosensitive body unit. If images are formed already on the photosensitive body unit at that time, it is not favorable because the images are blurred due to the irregular rotation that may occur where the images are already formed on the photosensitive body unit.
(b) As a means for cleaning the remaining toner on the photosensitive drum, it is a general method in which a blade shaped rubber is made in contact with the surface of the photosensitive body to wipe the toners. Where the toner is remaining on the photosensitive body surface, the toner itself becomes a lubricant, so that there is no problem of friction between the cleaning means and the photosensitive body. If no toner is remaining on the photosensitive body surface, wearing on the photosensitive body surface layer proceeds due to friction between the photosensitive body and the cleaning means, thereby reducing the duration of the photosensitive body.
Accordingly, it is unfavorable to operate rotating the photosensitive drum in a state not carrying the toner more than needed.
(c) In a case of a multicolor image forming apparatus, image defects such as color deviations may occur if the image starting positions of the respective colors are not aligned accurately. As a means for aligning the starting positions of the respective colors, a marking for detecting a position is generally formed on the intermediate transfer belt, and an image is formed with reference to the marking.
In the image forming apparatus, a means detectable of the marking position is provided, but the timing starting the image is not yet known until when the marking passes by the detecting means. In order to let the user send a print signal and to print out the material as early as possible, it is desirable to pass the marking portion on the intermediate transfer belt by the detecting means as early as possible. If the marking portion passes with delay, however, the timing starting image formation may be delayed, and images may be outputted with delay.